The elimination of inductive coupling is an important step for the use of RF coils arrays for MRI, such as in parallel imaging for MRI. For example, in nuclear magnetic resonance (NMR) imaging, if some effective mutual inductance remains among the coils in the coil array, the NMR signal obtained from one coil may disturb the flux in another coil, which may make it difficult to match and tune each circuit with a coil to the input impedance of the respective preamplifier circuit.
A common method to isolate the coils in the coil arrays avoids the build up of significant currents from the NMR signal among each of the coils in the coil array, in such way that the effects from the mutual inductances may be neglected. This may be conventionally achieved by connecting each coil to a circuit that should behave as an open circuit from its input port. A common method employed for this circuit implementation usually involves the use of a network transformer (or an impedance transformer) and preamplifiers with very low input impedance (typically <2Ω) and decoupling networks with lumped elements. However, the construction of low noise figure preamplifiers with extremely low input impedance is not easy to accomplish. In addition, the use of preamplifiers with low input impedance imposes technical restrictions on coil design, especially when considering geometries that require overlapping loops where there is a significant amount of mutual inductance between the RF coils.
In addition, it is known that the coupling of coils to the matching network is usually a hard task to accomplish and is highly dependent on effects caused by sample loading. One such effect is produced by voltage differences in different parts of the coil that generate an electrostatic field around the coil. This electrostatic field may couple to the sample, causing dielectric losses and consequently a reduction of the received signal. Another undesirable effect is caused by standing waves that may be present in the cables that connect the coils to the preamplifier, which may feed back to the pickup coil or may also represent a radiation loss for the NMR signal. Conventionally, these disturbances in the signal-to-noise ratio may be improved by reducing the dielectric losses and also by reducing the currents in the ground loops that may provide resistive and radiation losses.
Another technique employed to decouple RF coils in an array attempts to cancel the flux between any two coils in the array. However, canceling flux is not an efficient method for isolating non-adjacent neighboring coils. Further, canceling flux tends to degrade the NMR signal by inevitable insertion of losses on coils and, moreover, may not work for arbitrary coil geometries.